Ice dam forming on the roof — Oakland, California

An ice dam is a ridge of ice that forms along the eaves of a roof, preventing snowmelt from draining off. Heat escaping through the attic melts snow on the upper roof, and the water refreezes when it reaches the cold overhang. As the dam grows, water backs up behind it, seeps under shingles, and leaks into the attic, ceiling, and walls. Chronic ice dams signal inadequate attic insulation, poor ventilation, or air leaks from the living space into the attic. A roofer can install ice-and-water shield membrane on the vulnerable eave area, improve attic ventilation with ridge and soffit vents, and coordinate with an insulation contractor to seal bypasses that let warm air reach the roof deck.